Main features of commercial TiO2 samples.
This chapter presents the use of a commercial micro-sized TiO2 powder as an alternative to the traditional nano-powders as semiconductors in photocatalytic processes. Results of the photocatalytic efficiency towards the photodegradation of the traditional pollutant molecules both in gas phase (nitrogen oxides (NOx) and volatile organic compounds (VOCs)) and in water phase (phenol) are presented and compared to the results obtained with two nano-sized reference powders. Micro-sized TiO2 is also industrially coated at the surfaces of porcelain grés tiles (Active Clean Air and Antibacterial Ceramic™). The possibility to have a photocatalytic material, strongly stuck at the surface of a vitrified tile, increases the use of photocatalysis in real conditions: no problem of filtration of the semiconductor from the liquid medium after use and no risks of leakage of nanoparticles in the atmosphere. Tests were performed using reactors equipped with UV-A lamps and with suitable analytical systems, depending on the final purpose. Characterization data from both powders and coated tiles are put in correlation with the photocatalytic results to understand the semiconductor action during the photocatalytic process. Polluting molecules were chosen in order to cover all the common aspects of environmental pollution: NOx and some VOCs represent the model molecules to test the efficiency of the micro-sized TiO2 (degradation from the pristine molecule to CO2 or inorganic salts) in gas phase. As for the water pollution, phenol was chosen as common pollutant in worldwide rivers. Moreover, tests on self-cleaning and antibacterial properties are also reported. The positive results of micro-sized TiO2 both in powder and coated onto the surface of porcelain grés tiles open the way to new photocatalytic products that do not make use of nanoscale powders avoiding problems to human safety caused by the inherent toxicity of the nanoparticles.
- Microsized TiO2
- photocatalytic tiles
- oleic acid
- Escherichia coli
- Staphylococcus aureus
- methicillin-resistant MRSA
The importance of photocatalysis is related to the increase of the environmental problems regarding climate changes, scarcity of energy and greenhouse gases emissions, which increase the need of new environmentally beneficial technologies. Photocatalysis can also be used for water purification: many references are already present in literature on the degradation of dyes or drugs in aqueous solutions simulating the real pollution of rivers and lakes.
TiO2 is very stable and very suitable for applications in environmental improvement and protection. Other than the antipollution properties, TiO2, always in the anatase phase, is capable of conferring both self-cleaning and antibacterial properties to the surfaces .
Titanium dioxide is a semiconductor widely used in nanotechnology . Since Fujishima and Honda discovered the phenomenon of photocatalytic water splitting , effort has been devoted to the research of TiO2 materials, with potential applications in several fields, from biomaterials to self-cleaning processes and from sensing to photovoltaic applications, as well as in photocatalysis [4–6]. Indeed, it has been broadly studied for environmental applications, such as pollutant abatement both in liquid and in gas phase, and it has many advantages compared to other photocatalysts in particular because of low production cost and high chemical stability .
It is possible to find TiO2 in three different crystallographic phases: anatase, rutile and brookite. Anatase is the most photocatalytically active; it has a bipyramid tetragonal symmetry, with four units per elementary cell (Figure 1a), and its structure consists in elongated octahedra. On the contrary, rutile is largely used as pigment and in the coating industry, precisely because of its chemical inertness; it is tetragonal, with two formula units per elementary cell (Figure 1c). Brookite is a metastable phase, and apart from a limited range of temperature and pressure, it easily turns in the other two forms; it has got an orthorhombic symmetry, with eight formula units per elementary cell (Figure 1b) .
Powdered TiO2 is used as a common white pigment and effective opacifier thanks to its brightness and very high refractive index. More than 5 million tons of pigmentary TiO2 are produced annually worldwide, and this amount is expected to increase as consumption continues to rise . It is often used to provide whiteness and opacity in commercial products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e. pills and tablets) as well as most toothpastes .
The use of nanomaterials (NMs) such as titanium dioxide in the production of cosmetics, sunscreens, surface coatings and some food products is widespread and successfully applied , as well as the most part of NMs has been used in products for health and fitness, including both medical/pharmaceutical and personal care products .
New physical and chemical properties emerge when the size of the material reaches the nanoscale. Firstly, the specific surface area increases dramatically as the size of a material decreases , and the high surface area brought about by small particle size is beneficial to many TiO2-based devices, as it facilitates the interaction between the devices and the surrounding environment.
For this reason, in recent years nanotechnology has spread through all sectors of science and technology including engineering, medical, pharmaceutical, agriculture, environment and many more.
However, nanoparticles exhibit potential risks in terms of: (i) dispersibility, that is, the ability to disperse in the environment including potential long range transport; (ii) ecotoxicity, i.e. the ability to cause adverse effects to organisms in the environment; (iii) persistency, that is the ability to remain in the environment; (iv) bioaccumulation, that is the ability to either bioaccumulate or bioconcentrate in higher order organisms; (v) reversibility, i.e. the ability for removal or to reverse their original introduction from environment.
Human health risks associated with nanotechnology include both inhalation and/or exposure of/to nanoparticles or the ingestion of water or food and dermal contact . The exposure to nanoparticles leads to a number of effects including oxidative stress, lipid peroxidation, genotoxicity, lung diseases, inflammation or pulmonary pathological changes [15–18].
In literature, there are many references that describe and present information on titanium dioxide health risks . The International Agency for Research on Cancer (IARC) classified ultrafine TiO2 as possible carcinogenic to humans . The USA National Institute for Occupational Safety and Health reported that the exposure to ultrafine TiO2 should be considered potentially carcinogenic  and the Federal Institute for Occupational Safety and Health showed some results of toxicokinetic analysis in lungs and in organs like liver and brain, finding a small solubility effect under physiological conditions, as well as lung tissue inflammation for exposures to high dose of TiO2.
Thus, despite the good properties and the high efficiency of nano-sized TiO2, these concerns over safety may limit its applications, and it is very important to find a replacement.
For this reason, the use of micrometric TiO2 becomes more and more interesting and few years ago some investigations demonstrated the possibility to use this material for the photodegradation of pollutant molecules, such as volatile organic compounds (VOCs), nitrogen oxides (NOx) or organic molecules that mainly pollute the water systems [23, 24]. It has also been successfully used in building materials to exploit self-cleaning and antibacterial effects .
Generally, micro-TiO2 is usually used as a pigment but never in photocatalysis; however, as already mentioned, testing it for the photodegradation of different kinds of pollutants, both in gas and in aqueous phase, the efficiency of this material was confirmed at this point [1, 10, 24]. Furthermore, application of micro-TiO2 in different materials, such as building materials and paintings, may bring many benefits, because when compared with nanometric TiO2, it is easier to handle, cheaper as well, and shows a good photoefficiency .
The coating of TiO2 allows conferring the material surfaces all the properties of TiO2 as semiconductor . It is already widely used on different materials, for different applications: on exterior construction materials [27, 28] for water  and air purification , for the preparation of self-cleaning and antibacterial cements , on tiles [23, 31], on glass , and, recently, for the protection of the cultural heritage . Most of these materials are currently prepared with nanometric titanium dioxide, with all the ensuing problems described above.
Different techniques can be used to prepare and coat TiO2 on solid supports, such as glass, metals or ceramics; to improve both adhesion and hydrophilicity of coating, silica can be added as a suspension, while several coating techniques, such as dipcoating, spincoating and spraycoating, can be used to spread the TiO2 mixture over the support.
Among the several kinds of supports to obtain TiO2-coated materials, with interesting photocatalytic properties that can be used in both outdoor and indoor environments, porcelain grés tiles are very attractive. When the industrial production began in the 1970s, porcelain grés tiles were aesthetically not very attractive and considered as just a technical material characterized by lack of porosity and strong resistance to both abrasion and acid attack. However, in the last decades, porcelain grés tiles underwent significant transformations in terms of appearance, mainly in size. Currently, thanks to advanced and innovative industrial production methods, both properties and appearances of these materials are improved and completely fit the market requests. In particular, the possibility to prepare slabs of large sizes (such as 300×150 cm) is the new frontier of building materials . Porcelain grés tiles are manufactured under high pressure by dry-pressing fine processed ceramic raw materials with large proportions of quartz, feldspar and other fluxes and are characterized by a very low water absorption rate (less than 0.5%). In fact, these materials are fired at very high temperatures (1200–1300°C) in kilns . A complete frost resistance due to the lack of porosity, complete waterproofing, durability, hardness and wear resistance properties thus characterizes the final ceramic product.
Besides these noteworthy architectural features, the deposition of anatase TiO2 form converts the traditional ceramic into a photocatalytic eco-active material with the ability to reduce polluting molecules present in both air and water, to eliminate bacteria and to reduce the surface dirt thanks to the self-cleaning properties .
Self-cleaning refers to the pollutant degradation and dirt removal from surfaces, by their different behaviour towards water, relating to their hydrophobicity or hydrophilicity . The main characteristic of superhydrophobic surfaces is high roughness, which implies low adhesion forces between contaminating particles and the surface, so water droplets roll off very quickly , and dirt can be very efficiently removed. On the contrary, TiO2 confers the superhydrophilicity feature to the surfaces, under light irradiation, and water dropped onto titanium dioxide forms low contact angle. The Research Institute of Toto Ltd discovered the effect for a photocatalytic surface irradiated by sunlight in 1995.
There are two different ways to exploit the TiO2 features for self-cleaning: (i) in the first case, TiO2 nanoparticles are introduced inside a polymeric matrix in order to create nano-roughness on the surfaces and make them hydrophobic and superhydrophobic [38, 39]; (ii) secondly, the hydrophilicity of TiO2 is the key property. Indeed, under UV exposure, titanium dioxide becomes super hydrophilic, and the decrease of the water contact angle creates a uniform water film on the treated surfaces, preventing contact between external dirt and surfaces themselves . The main application is on building coverings because it decreases the periodicity of cleaning or renovation of the surfaces, maintaining their aspect as constant as possible .
The antibacterial properties are also important and interesting. They allow destroying the bacteria present at the photocatalytic material surfaces with the only action of light and humidity present in the air without using specific detergents, source of pollution for rivers and seas [42, 43]. Many bacteria have been investigated, and the photocatalytic antibacterial activity is always confirmed even with the most dangerous MRSA species, resistant to antibiotics .
In this chapter, we show two new frontiers in photocatalysis. Firstly, the real possibility to use a commercial micro-sized TiO2 in powder form (K1077 by Kronos) as a photocatalyst is reported, with results compared with two traditional nanometric powdered samples (P25 by Evonik and PC105 by Cristal). Secondly, we report the industrial preparation of a photoactive porcelain grés tile, coated by spray method with the micro-sized TiO2 (White Ground Active sample). In both cases, characterization results of both powders and tiles will be put in correlation with all the photocatalytic properties: antipollution in water (phenol) and air (NOx and VOCs), self-cleaning (oleic acid as reported in the standard ISO 27448 rule) and antibacterial investigating the degradation of Escherichia coli and Staphylococcus aureus MRSA.
The possibility to use micro-sized TiO2 in a commercially manufactured product opens the way to a new generation of intrinsically safer materials than the traditional photocatalytic products for both workers in the factories and public safety.
2. Material and methods
Three commercial TiO2 samples were selected without further treatment: two nano-sized by Evonik (P25) and by Cristal (PC105) and one micro-sized by Kronos (K1077).
Staring from the traditional porcelain grés tiles fired at 1200°C with a vitrified surface, photoactive porcelain grés tiles by GranitiFiandre S.p.A were prepared covering the surface with an aqueous mixture of pure anatase micro-TiO2 in powder form (K1077) and a commercial SiO2-based compound by spray method in airless cabins. To ensure the semiconductor strong adherence and the complete surface vitrification, given by the presence of SiO2, tiles were heated at 680°C for 80 min in an industrial kiln and then brushed to remove TiO2 particles present at the ceramic surface and not completely stuck (sample name: White Ground Active, i.e. WGA) [21; Patent n. EP2443076].
2.1. Sample characterization
We used the following equipments to characterize the powdered samples. N2 adsorption/desorption measurements at 77 K Brunauer–Emmett–Teller method (BET) using Sorptometer instrument (Costech Mod. 1042) to investigate specific surface area (SSA) of all samples.
X-ray diffraction (XRD) with a PW3830/3020 X’Pert diffractometer from PANalytical working in Bragg-Brentano geometry (Cu Kα1 radiation; k = 1.5406 Å) was chosen to inspect the crystalline nature of the samples.
High-resolution transmission electron microscopy (HR-TEM) using a JEOL 3010-UHR instrument (acceleration potential: 300 kV; LaB6 filament) was used to detect the fine morphology of the materials (powder form). Samples were always ‘dry’ dispersed on lacey carbon Cu grids.
X-ray photoelectron spectra (XPS) were obtained in an M-probe apparatus (surface science instruments) using a monochromatic source of Al Kα radiation (1486.6 eV).
For the band-gap determinations, diffuse reflectance spectra of the materials in powder form were recorded employing a UV-Vis scanning spectrophotometer (PerkinElmer, Lambda 35 model), equipped with a diffuse reflectance accessory. The Kubelka-Munk function was used to elaborate data to determine the band gap values by performing the first derivative of the Kubelka-Munk equation.
In situ FTIR spectroscopy was used to detect the surface functionalities, mainly surface hydroxyl species. Absorption/transmission IR spectra were collected at room temperature (RT) on the samples outgassed in vacuo at RT by means of a Perkin-Elmer FT-IR System 2000 spectrophotometer equipped with a Hg-Cd-Te cryo-detector. Spectra were collected in the 7200– 580 cm−1 range at 2 cm−1 resolution. For this analysis, the powdered photocatalyst was pressed in self-supporting pellet (~10 mg cm−2), placed in a homemade quartz cell, equipped with KBr windows, connected to a conventional vacuum line.
High-resolution SEM-EDX investigations were performed on ceramic tiles using a field emission gun electron scanning microscopy LEO 1525, while the elemental composition was obtained using Bruker Quantax EDS.
2.2. Photocatalytic tests
2.2.1. VOCs (volatile organic compounds)
The photodegradation of some VOCs (i.e. acetone, acetaldehyde, ethanol and toluene) was monitored conducting the degradation in a Pyrex glass cylindrical reactor with diameter of 200 mm and effective volume of 5 L . For each test, 0.05 g of photocatalyst (in powder form deposited on flat glass disk from a 2-propanol slurry) was used. Hot chromatographic air, humidified at 40%, and a fixed amount of the volatilized VOC were mixed to obtain the requested gaseous mixture in the reactor. The actual concentration of the organic molecule in the reactor was determined directly by micro-GC sampling as shown in Figure 2. Photon sources were provided by a 500 W iron halogenide lamp (Jelosil, model HG 500) emitting in the 315–400 nm wavelength range (UV-A) at 30 Wm−2. The duration of each test was selected by considering the difficulty in degrading a molecule, in a time between 2 and 6 hours. The intermediate oxidation products were determined directly by investigating the sample surface after the kinetic run by FTIR spectroscopy and the determination of CO2 was performed by gas chromatography as well.
• Static condition
The photocatalytic activity of both powder samples was tested in NOx degradation, operating in static conditions in a Pyrex glass cylindrical reactor with an effective volume of 25 L . The amount of catalyst used in the tests in this case was 0.05 g.
NO2 (0.6% in nitrogen) was mixed with air humidified at 40% to obtain the gaseous mixture in the reactor. It is important to underline that we start from an inlet gas of pure NO2 pulsed into the reactor that, as soon as it comes into contact with the air present inside the vessel, reaches the chemical equilibrium between NO and NO2 so as to naturally obtain the NO and NO2 mixture in air. Different concentrations of NOx in the reactor were chosen: i) 1000 ppb in order to follow the same pollutant concentration requested by the ISO 22197-1 rule; ii) 200 ppb that is very close to the alert threshold set by the EU Directive 2008/50/CE for NO2. Photon sources were provided by a 500 W iron halogenide lamp (Jelosil, model HG 500) emitting in the 320–400 nm wavelength range (UV-A), but with a specific UV power on the surface of the samples set at 10 Wm−2. The NOx photocatalytic tests were performed at 30°C and lasted for 4 or 6 hours. The actual concentration of pollutants (NO, NO2 and consequently their sum, i.e. NOx) in the reactor was determined in this case directly by a chemiluminescence (Teledyne, Mod. 200E).
• Continuous flow reactor
The photoefficiency towards the NOx degradation of tiles of 600×600 mm was confirmed using a purpose-built reactor . The continuous flow reactor (Figure 3) has walls of 10 mm of thickness and an internal size of 625×625×115 mm with four inlets and one opposite outlet and can house a sample of 600×600×10 mm. This configuration assures a good homogeneity of the reactants in the gas phase and a contact between reactant and photocatalytic tiles that well reproduces the real working conditions.
Reactor is equipped with a thermo-hygrometer model HT-3006A to measure both temperature and humidity during the tests. Humidity inside the reactor is maintained constant between 40–50% through bubbling the gas flow into a gas bubbler (containing water and saturated aqueous vapour). The irradiation was emitted by two iron halogenide lamps (Jelosil, model HG 500) positioned at 770 mm centre-to-centre distance. The light intensity incident on the sample surface in the UV-A region was measured with a Delta Ohm radiometer and regulated at 20 Wm−2 by adjusting the distance of the lamp from the reactor. The degradation was performed at different initial NOx concentrations, obtained by diluting the stream of NOx from the cylinder with air, at RT and working with a total gas flow of 140 and 180 NL h−1. The concentration values were chosen to work closely to the limit values reported on the Directive 2008/50/EC, in particular, 106 ppb (equal to 200 µg m−3, value not to be exceeded more than 18 times in a calendar year) and 213 ppb (400 µg m−3, alert threshold).
2.2.3. Liquid phase
Regarding photocatalytic tests in liquid phase, the experimental set-ups are different depending on whether we use powders or tiles.
• Photocatalytic test using powdered nano- and micro-TiO2 catalysts
Dye degradation in water media was performed in a slurry reactor, equipped by an internal refrigerating serpentine system (Figure 4a) of 0.5 L volume . Irradiation was allowed by the use of an external UV-A lamp (500W, Jelosil®, HG500, halide lamp), emitting in the range 315–400 nm and with a power evaluated by a radiometer instrument (Delta OHM, model HD2102.2) of 75 Wm−2. A UV-Vis spectrophotometer analyzer (T60 UV-Vis PG LTD instrument) has been used to measure the concentration of dyes or organic molecules as a function of time.
• Photocatalytic test using photoactive TiO2 tiles
A cylindrical batch reactor of 1 L volume, refrigerated with a cooling jacket, was used for dye degradation tests in the presence of 10 photoactive tiles (0.03 m2 total surface photoactive area) immersed into the liquid solution, as schematically shown in Figure 4b . In this case, two different lamps directly immersed into the aqueous solution were used: a typical germicidal W UV lamp (Philips TUV BL-S, model AEPL-7913 mercury vapour low pressure), with an UV-A illuminance at the tile surface of 1 Wm−2 and a 125 W UV-A lamp (Jelosil, mercury vapour low pressure), with an illuminance of 65 Wm−2.
The ISO 27448 rule was precisely followed to perform the measurements on tiles. The self-cleaning action is assessed by measuring the contact angle generated by the film of pure oleic acid and by monitoring the changes in the angle due to any degradation of the deposited acid due to the UV irradiation. Only if the surface has photocatalytic properties, the changes of the contact angle value occur.
The contact angle was measured (Kruss instrument equipped with high resolution TV camera) after the cleaning pre-treatment by UV irradiation, after contact with the oleic acid (zero time) and after irradiation with UV lamp at 20 Wm−2 at 2, 4, 6, 24, 48, 72, 74 and 76 hours. At each time, the measurement is repeated on five random points on the surface of the tested material.
Samples of WGA (size 100×100 mm), randomly taken from an industrial production batch, were placed in a suitably prepared solution containing 0.5 vol% of oleic acid (Fluka reagent, purity >80%) in n-eptane (Fluka reagent, purity >99%), in order to obtain a uniform coating of oleic acid at the surface. The amount of oleic acid thus applied resulted to be 2.0 ± 0.2 mg, (value measured by means of a Gibertini Elettronica precision balance). An additional sample of WGA was pre-treated and then coated with the oleic acid using the same method. At the end of this procedure, the additional sample was then placed in a vessel in the dark with controlled air and humidity, throughout the duration of the whole test (76 hours).
2.2.5. Antibacterial efficiency
The measure was performed on photocatalytic tiles following the ISO 27447:2009 rule (film adhesion method) (rif. ISO 27447:2009 Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics)—Test Method for Antibacterial Activity of Semiconducting Photocatalytic Materials, 2009).
The test used E. coli ATCC 8739 and S. aureus ATCC 43300 methicillin-resistant MRSA. Each strain was inoculated into a nutrient agar slant, incubated for 16–24 hours at 37°C ± 1°C and subsequently transferred in a new nutrient agar slant and again incubated at 37°C ± 1°C for 16–20 hours. An adequate quantity of bacteria was dispersed in 1/500 NB (nutrient broth) to obtain a count of 6.7 × 105 to 2.6 × 106 cells/ml. Samples of 50×50 mm square was rinsed with distilled water and autoclaved at 121°C for 30 minutes before the test to remove any organic residue present at the ceramic surface. For each bacterium strain were tested specimens without antibacterial treatment and six specimens with antibacterial treatment; 0.15 ml of bacterium suspension was put onto each specimen and covered with a 40×40 mm square inert and non-water adsorbent film with a transparency rate over 85% for the 340–380 nm range. Each specimen was placed in a 100 mm diameter Petri dish containing moistened paper filter to prevent drying out of the suspension covered with a 1 mm thickness borosilicate glass with a transparency rate over 85% for the 340–380 nm range.
The samples were irradiated with a fluorescent UV lamp (18W Philips PL-L) with an intensity of 0.25 mWcm−2 for 8 hours. A similar sample was treated with the same procedure but left in the dark for the whole test. A viability count was performed by dilution and plating on nutrient agar (NA) incubated at 37°C for 48 hours .
The bacterium concentration of the washout liquid is obtained by Eq. 1 and expressed to two significant digits:
where P is the bacteria concentration (cells/ml); Z is the average number of colonies in two Petri dishes; R is the dilution factor. When the number of viable bacteria is less than 30 in the Petri dishes with 1 ml of washout solution, the cell number is used to calculate the average number.
where N is the number of cells of viable bacteria; P is the bacteria concentration obtained in Eq. (1) (cells/ml); V is the volume of Soybean-casein digest broth with lecithin and polysorbate (SCDLP) medium for washout (ml).
where RL is the photocatalyst antibacterial activity value, after UV irradiation of intensity L; L is the UV irradiation intensity (mWcm−2); A is the average number of viable bacteria of non-treated specimens, just after inoculation; BL is the average number of viable bacteria of non-treated specimens, after UV irradiation of intensity L; CL is the average number of viable bacteria of photocatalytic treated specimens, after UV irradiation of intensity L.
where ∆R is the photocatalyst antibacterial activity value with UV irradiation; BD is the average number of viable bacteria of non-treated specimens, after being kept in a dark place; CD is the average number of viable bacteria of photocatalytic treated specimens, after being kept in a dark place.
3. Results and discussion
3.1.1. Commercial powders of TiO2
The XRD patterns relative to the three selected samples are reported in Figure 5. Anatase is the unique polymorph present for all samples, except for the well-known P25 material, which exhibits the 80:20 anatase/rutile ratio phase composition. The crystallographic reflexes (1 0 1), (2 0 0) and (2 1 1) have been used to calculate the average crystallite size of the various titania particles (Table 1, third column). P25 and P105 possess comparable crystallite size centred at around 25 nm, while the sample K1077 exhibit a value of 130 nm. These features are reflected in the BET surface area of the micro-sized sample (K1077), which is much lower compared to the nano-sized ones (Table 1, second column).
|Sample||BET surface area (m2g−1)||Average crystallite size (nm)||XPS Ti 2p3/2 (eV)||XPS OH/Otot||Band gap (eV)|
|P25||50||26||458.4 ± 0.1||0.14||3.21|
|PC105||80||23||458.4 ± 0.2||0.85||3.19|
|K1077||12||130||458.3 ± 0.1||0.32||3.15|
A comparison between the morphological features exhibited by the nano-sized powders, respectively known as P25 and PC105, and the micro-sized K1077, was carried out by TEM. TEM images (Figure 6) confirm the above average crystallite size extrapolated by XRD analysis also excluding the presence of ultra-fine particles in K1077. It can also be evidenced that the nano-sized materials perfectly fall within the ‘nano’ definition: in fact, both nano-samples are characterized by average particles size of 15–30 nm; they exhibit a closely packed nature and almost roundish contours. However, the highly crystalline nature of these powders is confirmed by the high incidence of fringe patterns, which are generated by the superimposition of crystals almost all belonging to the TiO2 anatase polymorph. In both cases, the prevalently observed planes (d = 0.357 nm) are ascribable to the (1 0 1) family (ICDD anatase file no. 21-1272).
As far as the micro-sized sample is concerned, much larger average dimensions of the particles (Figure 6) are observed. This feature is in total agreement with the indications obtained by means of XRD analysis, evidencing the average dimensions in the 120–200-nm range for the K1077 sample. Moreover, also in this case the most frequently observed crystal planes are those belonging to the (1 0 1) family of the TiO2 anatase polymorph, thus exhibiting a much less defective nature of the particles.
XPS was used to detect the surface state of the TiO2 particles by (Figure 7). No significant differences can be appreciated among all the investigated samples in the Ti 2p region concerning both binding energies (BE) and full width at half-maximum (FWHM) values. The Ti 2p3/2 peak is always present as a single doublet and the BE (forth column in Table 1) compares well with the data for Ti(IV) in TiO2 . The analysis of the oxygen peaks exhibits the presence of two different components, which can be attributed to lattice oxygen in TiO2 (529.9 eV) and to surface OH species (>531.5 eV), respectively . For PC105, a particular O1s shape was observed (Figure 7b): in fact, the OH component is very intense probably due to a particular industrial synthesis in order to enhance the photocatalytic efficiency of the sample.
The hydrophilicity/hydrophobicity character of photocatalyst surface plays a crucial role in determining the adsorption step and thus the photocatalytic activity in the degradation of pollutants . The OH/Otot surface ratio, reported in the fifth column of Table 1, is a quantitative measure of the hydrophilicity/hydrophobicity of the TiO2 surface, estimated using the XPS-determined surface OH atomic concentrations normalized by the total oxygen atomic concentrations (Otot) . PC105 exhibits the highest concentration of OH that represents the 85% of the oxygen at the surface as previously underlined. It is noteworthy that the micro-sized sample presents a higher atomic concentration of OH groups in comparison with P25, pointing out the higher hydrophilic character of its surface.
From XPS spectra, we can obtain both the information on the BE of a specific element and the total density of states (DOS) of the valence band (VB) . This measurement is useful to unravel the effect of TiO2 structural modifications on the electronic properties of the material. Comparing semiconductor nanoparticles/microparticles, the quantum size effect could also play a role, giving band-gap narrowing in the bigger-sized particles in respect to the smaller ones. Figure 8 reports the VB XPS spectra of both nano-sized samples and the micro-sized one. The VB maximum position was determined through linear extrapolation and the same values around −2.6 eV were measured for all samples, which is a typical value for TiO2 .
Experimental data of diffuse reflectance were elaborated to absorption coefficient values F(R) according to the Kubelka-Munk equation (Eq. 5),
where R is the reflectance.
The corresponding band gap values obtained by this procedure for all samples are reported in Table 1 (sixth column). The obtained band gap values do not exhibit large differences among the various samples and fall in the range expected for the TiO2 material.
3.1.2. Photocatalytic active tiles
Basic features of the porcelain grés tiles were determined after the coverage with the photo-catalytic mixture to verify that the coating procedure does not alter the ceramic body. Lack of porosity, resistance to surface abrasion and durability were measured by water absorption , rotation of an abrasive load on the tile surface  and determination of frost resistance , respectively. The frost resistance determination is an intrinsic measurement of the product durability.
The coating with the SiO2-based compounds and the following calcination step preserve the pure anatase form verified by XRD measurements . The presence of SiO2, together with TiO2, enhances the formation of hydroxyl radical •OH, which would be achieved via strong Brønsted acid sites at the TiO2/SiO2 interface region . This incorporation inhibits the crystal growth of TiO2 allowing the preservation of the anatase structure at 680°C .
SEM images of WGA show a homogeneous distribution of the micro-sized TiO2 crystallites without ultrafine particles (Figure 9), on the porcelain tile surface. If measurement is carried out at higher magnification (Figure 9, at the bottom), it is possible to observe that the particle dimension falls in the 100–125 nm range, while at lower magnification it is possible to evidence larger particles of SiO2.
Wettability measurements were performed on both a grés tile and a photocatalytic active tile (Figure 10), exhibiting a completely different surface; in fact, the photocatalytic WGA tile presents very hydrophilic surface features with an average angle of 23.0°, which can be explained by the super hydrophilic property of TiO2 photocatalyst .
3.2. Photocatalytic tests
3.2.1. Nano- and Micro-TiO2 commercial samples
The results obtained with the commercial catalysts starting from 1000 ppb of NOx are reported in Figure 11, and it is well evident that all samples show good photocatalytic performances. In particular, after 60 minutes of reaction, the NOx conversion is almost complete. From this point, the actual difference between the nano-powder and the micro-one is evident, because while P25 reaches a complete degradation of NOx, K1077 is not able to degrade the entire NOx amount, even if it is able to convert more than 90% of the pollutant approximately. Thus, even if the nano-sized material exhibits the best performances, the photocatalytic activity of the pigmentary sample is very good, in agreement with the presence of appreciable amount of surface hydroxyls.
Moreover, the results obtained using lower NOx concentration show that nano-sized and micro-sized powders have a similar photocatalytic activity. Both samples reach the complete NOx degradation within 50 minutes. Testing the materials in more diluted initial concentrations, the difference between micro and nano-samples is highly limited, and this means that the amount of surface OH species on the K1077 is enough to guarantee performances similar to that of P25, as also evident in Figure 12, relative to the FTIR features typical of OH species present at the surface of the three TiO2 systems, after evacuation at RT.
Nano- and micro-sized commercial sample photoactivity has been studied also for the degradation of different VOCs (acetone, acetaldehyde, toluene and ethanol), as shown in Figure 13 for the K1077 sample. VOC molecules are very interesting to study because they are mainly responsible for both outdoor and, even more, indoor pollution [60, 61].
Acetone reaction pathway passes through the formation of acetaldehyde as a by-product (Figure 14a) as reported by Stengl et al., while toluene is a less polar and less hydrophilic VOC, with a degradation that is more difficult because of the presence of the aromatic ring. Moreover, the degradation pathway of toluene passes through many by-products, which are not so easy to degrade and that in most cases are adsorbed on the TiO2 surface (Figure 14b).
The nano-sized samples exhibit higher photocatalytic efficiency, leading to the complete pollutant degradation within a shorter reaction time, than the micro-sized K1077 sample. For acetone, acetaldehyde and ethanol, using both nano- and micro-sized powders, CO2 is the only final product of the reaction, confirming the complete degradation without the formation of by-products adsorbed on the sample surface (verified by FTIR measurements on the sample surface after the kinetic runs). The case of toluene is different: for all samples, the pristine pollutant is not completely degraded even after 6 hours of reaction, and most of the by-products are not completely degraded, as well.
Considering surface area and particle size, the VOC photodegradation kinetic is faster when the particle crystallite size is smaller. The nature of the reactive sites is influenced by the morphological features of crystallites, thus inducing different type of exposed OH reactive sites onto the surface of nano- and micro-sized TiO2 samples. Despite the low surface area, K1077 presents an abundant OH radicals’ population, thus it shows to be able to photo-degrade several VOCs, with not excessive differences in terms of performance compared to nano-TiO2 powders.
3.2.2. Photocatalytic active tiles
Micrometric titanium dioxide can be used with very good results for the photocatalytic abatement of pollutants, such as NOx or VOCs. Thus it has been industrially applied on porcelain grés tiles with many advantages in terms of (i) ease of manipulating micrometre powders instead of nano-powders, (ii) cost, because pigmentary TiO2 is cheaper and (iii) safety, because of the toxicity of TiO2 nanoparticles that can be inhaled.
The photocatalytic active tiles, obtained after the TiO2 coating as described above, have been tested in the photodegradation of both gas and liquid pollutants, in particular studying their photocatalytic performances for the photodegradation of NOx in gas phase, and for the photodegradation of phenol in liquid phase. Particularly, nitrogen oxides are the worldwide air pollution reference.
When tested for the NOx abatement, photocatalytic active tiles show very good results. The pollutants are successfully degraded and the maximum conversion is reached just after about 40 minutes. The experiments of this run were performed at 140 NL h−1 and with a starting concentration of 100 ppb of NOx (Figure 16). These experiments were conducted changing the parameters, in particular the NOx starting concentration, as well as the duration of the reaction, obtaining in all cases good results .
Investigating the phenol removal in water in the presence of WGA, it was observed that the trend of the degradation processes depends on the pollutant starting concentration. After 6 hours of exposure, the phenol photodegradation percentage is almost the same at 15 and 25 ppm (78 and 73%, respectively), and much lower at 50 ppm (46%), as shown in Figure 17.
The mineralization data show that at the lowest concentrations, the pollutant adsorption is followed immediately by its degradation and mineralization; on the contrary, at higher concentrations, the mineralization process starts later, and it is related to the hydroquinone formation, that is the main by-product (see Figure 18).
Therefore, the photocatalytic tile performances strongly depend on the starting pollutant concentration; anyway, photocatalytic tiles are able to degrade also the organic molecule, and they are effective in liquid phase.
These tests were carried out under UV-A light, but the real frontier will have to transform these materials active and efficient to sunlight and artificial light. The artificial lighting in the most part of the indoor environments is achieved with LED lamps, without any contribution of UV irradiation. Concerning this, many studies on the commercial micro-sized powders show that, modifying TiO2, it is possible to make it photoactive even to the visible wavelengths [62, 63]. This is very promising for a future application on porcelain grés tiles and for their application in the indoor environments lit by LED lights.
3.3. Self-cleaning tests
The surface of the WGA sample is coated with a film of oleic acid (Figure 19), following the method described in the experimental section. In the presence of UV irradiation, this substance is completely degraded, proving that the substrate has photocatalytic properties. Measurement was carried out according to the mentioned ISO Standard, by monitoring the change of the contact angle of a drop of water that fell onto the sample surface, which was illuminated by a UV-A source at known wavelength and power.
When between the beginning and the end of the test (76 hours), there is a variation in the contact angle, a photocatalytic material can be defined as self-cleaning.
The results of the measurements on WGA tiles are summarized in Figure 20 that shows two pictures taken and processed by the instrument used for the contact angle measurements.
Contact angle value of the WGA sample stored in the dark was also examined, but only at the end of the 76 hours. The comparison between the two tested slabs is listed in Table 2:
|Sample||UV||Original slab contact angle||Contact angle t0||Contact angle t48||Contact angle t76|
|WGA||Yes||31.3° ± 0.9°||68.5° ± 1.0°||41.0° ± 2.9°||30.1° ± 2.8°|
|WG A||No||31.3° ± 0.9°||67.8° ± 0.9°||—||68.2° ± 1.1°|
As shown in the table, the value of the contact angle gradually decreases from time t0 to t76, achieving the original value measured before the application of oleic acid onto the tile surface. This is due to the photocatalytic efficiency of the material, which is then able to degrade the oleic acid under UV irradiation .
The contact angle value at 72, 74 and 76 hours is constant, and it has returned to the value of the pristine ceramic surface at the beginning of the test, demonstrating that the oleic acid molecule has been completely degraded. On the contrary, there were no changes in the value of the contact angle of the sample of WGA treated with oleic acid but kept in the dark, confirming that natural degradation is not effective and that the photocatalytic process is needed to activate the photodegradation of the organic contaminant.
3.4. Test on antibacterial properties
Antibacterial tests performed on WGA samples confirmed the efficiency of the material to destroy bacteria present at the ceramic surface. Test results are reported in Table 3:
Experimental conditions of the test such as bacterial strains, temperature, humidity, irradiation, exposure time etc. are similar to the common condition of applications on the WGA. To evaluate the antibacterial activity of WGA, E. coli (Gram negative), as common inhabitant of human and animal intestine and therefore a suitable indicator of drinking water and quality food, and S. aureus (Gram positive) as human pathogen bacteria in particular the MRSA strain, characterized by some antibiotic-resistance to beta-lactam, have been chosen.
|Antibacterial activity of photocatalytic materials—materials||E. coli ATCC 8739||Staphylococcus aureus MRSA ATCC 43300|
|No. of bacteria inoculated||Cells|
|A—Average number of bacteria|
Non-treated specimens after inoculation
|BL—Average number of bacteria non|
-treated specimens after UV irradiation
|CL—Average number of bacteria photocatalytic|
treated specimens after UV irradiation
|RL—Photocatalytic antibacterial activity value|
after UV irradiation
|Reductions of bacteria from non-treated specimens|
after UV irradiation
|BD—Average number of bacteria non-treated|
specimens after being kept in a dark place
|CD—Average number of bacteria photocatalytic|
treated specimens after being kept in a dark place
|ΔR—Photocatalytic antibacterial activity value|
with UV irradiation
Test was performed at 25°C ± 1°C, temperature in the range of activity of the photocatalytic process and vitality of the microorganism tested both mesophilic. Test was conducted at pH 7.0 ± 0.2 (1/500 Nutrient broth), pH is an influential parameter for organism viability and a neutral pH near 7 is optimal for many biological processes. Test was performed in aerobic conditions (oxygen presence) to favour E. coli and S. aureus viability (aerobe/anaerobe facultative) at 0.25 mWcm−2 as normally indoor exposure beside the window in the daytime, for 8 hours of exposure time.
The antibacterial tests in this work passed all the evaluation conditions as:
1. The logarithmic value of the number of viable bacteria of non-treated specimens after inoculation derived from the equation: (Lmax−Lmin)/(Lmean) ≤ 0.2, where Lmax is the maximum logarithmic value of viable bacteria, Lmin is the minimum logarithmic value of viable bacteria, Lmean is the average logarithmic value of viable bacteria for three specimens;
2. The logarithmic value of viable bacteria of non-treated specimens after inoculation shall be within the 1.0 × 105 to 4.0 × 105 cell range;
3. The viable bacteria of non-treated specimens after light exposure shall be more than 1.0 × 103 cells for all three specimens;
4. After being kept in a dark place, the viable bacteria of non-treated specimens shall be more than 1.0 × 103 cells for all three specimens.
In the first test, the average logarithmic value of E. coli for three non-treated specimens after inoculation is 160,000 cells/ml, and this value increases to 600,000 cells/ml after being kept in a dark place, and a 170,000 cells/ml after 8 hours of UV irradiation. The average logarithmic value of E. coli for three treated specimens decreases to 130,000 cells/ml after being kept in a dark place, and a 10 cells/ml after 8 hours of UV irradiation.
In the second test, the average logarithmic value of S. aureus for three non-treated specimens after inoculation is 230,000 cells/ml, and this value increases to 400,000 cells/ml after being kept in a dark place, and 108,000 cells/ml after 8 hours of UV irradiation. The average logarithmic value of irradiation.
For both bacterial strains, an increase after being kept in a dark place on non-treated specimens is observed, but greater for E. coli than S. aureus, and a decrease was observed for treated specimens, but greater for S. aureus than E. coli. These differences may be due to the different structures of the cell wall of the two bacterial strains, and bacterial reduction is imputable to non-photocatalytic antibacterial activity which can be present in different intensities in ceramic tiles. Both microorganisms tested same decrease after UV irradiation is detected and WGA showed a photocatalyst antibacterial activity R greater than 2 (reduction greater than 99.99%); the antibacterial activity is similar for both Gram negative and Gram positive bacteria, including strains with methicillin resistance.
These results indicate that the action of reactive oxygen species (ROS), generated during the photocatalysis, on cell wall, cell membrane and internal cellular components, is very effective, and it does not seem to be affected by methicillin-resistant mechanisms. The same trend is observed for ∆R (photocatalyst antibacterial activity value with UV irradiation); the values obtained for E. coli and S. aureus (3.6 and 3.0) indicate a good photoefficiency on the WGA tiles.
Micro-sized TiO2 can successfully replace the nanometric powders for photocatalytic applications, as well as for the removal of dirt or bacteria from surfaces. The use of micrometric powders of TiO2 brings several advantages in terms of health safety, limitation of the problems related to the use of nano-powders in industries and cost.
From a chemical point of view, the commercial and micro-sized sample K1077 consists completely of anatase, which is the most photoactive crystallographic phase, as well as it exhibits a rather high population of hydroxyl radicals at the surface that confer an improvement of the performances in terms of pollutant oxidation. From an economic point of view, micro-TiO2 is much cheaper than the commercial nano-powders. In particular for high amount of powder, the replacement of the nano-powder by micro-powder leads to an extremely high cost saving. Moreover, micrometric TiO2 is overall efficient against all the different pollutants, with a not so high difference in performance compared to nano-TiO2, especially in pollution levels closest to the actual values.
The application of micro-TiO2 on porcelain grés tiles paves the way to a new scenario of products to improve the quality of life. Indeed, when TiO2 is coated on the porcelain grés tiles, all its properties can be fully exploited. Not only the photocatalytic tiles show self-cleaning features and antibacterial properties, but also they are able to degrade both organic and inorganic pollutants in gas and liquid phases. Particularly, the possibility to exploit the photocatalytic properties of tiles even in aqueous phase is a real innovation compared with most of the photocatalytic building materials, already available in the market.
To conclude, the use of micro-TiO2 is an actionable and necessary road, as well as the photocatalytic tiles coated with micro-powders are actually effective for a real improvement of our indoor and outdoor environments.
The work was partially financed by the project “Azione A del Dipartimento di Chimica per il Piano di Sostegno alla Ricerca – 2015/2017 - Linea 2” by the Dipartimento di Chimica, Università degli Studi di Milano.